Ok, kind of need to know the rating. But off hand, going with a Capacitor will do more than a secondary battery(and look nicer taking up less space) 1 farad per 1000 has always been ok in my book.
This is a in line battery and will get sucked down BEFORE your battery gets a draw. Having 2 battery's will provide more capacity at 12 volts but the draw will be applied to them both at the same time(making your lights dim).
Talk to your local audio shop they can help and tell you what you need. Off the top of my head guessing around $150 to $200 for the complete install. About the same cost, you would have been going with an extra battery.
Goodluck
